1. Field of the Invention
The present invention relates to a wireless communication apparatus.
2. Description of the Related Art
A wireless system uses a technique of transmitting a plurality of frames upon aggregating them to improve communication efficiency. For example, Task Group n (TGn), whose standardization has been under way via IEEE801.11, has proposed an aggregation technique of aggregating and transmitting a plurality of frames. This technique of aggregating and transmitting a plurality of frames can reduce overheads such as physical (PHY) and MAC layer headers accompanying each frame, which are required at the time of transmission and reception of frames, and the interval between frames.
On the other hand, as the frame length increases excessively, the wireless channel state estimated at the head of a frame differs from the state at the rear half portion of the frame, resulting in an error. That is, a proper frame length depends on the state of the wireless channel.
Existing IEEE802.11a/b/g also has studied an algorithm for link adaptation for controlling a transmission rate in accordance with a wireless channel. A conventional algorithm sets the initial value of a transmission rate to the minimum or maximum rate, and then starts control (M. Lacage, M. H. Manshaei and T. Turletti, “see IEEE802.11 rate adaptation: a practical approach”, Proc. Of ACM MSWiM, 2004, and reference 2: k J. C. Bicket, “Bit-rate Selection in Wireless Networks”, M. S. Thesis, MIT, 2005).
Consider admission control. According to conventional admission control, whether a new terminal can be accommodated is determined from the throughput (channel occupation ratio) at the current transmission rate of an existing terminal. In addition, JP-A 2003-251791 (KOKAI) discloses a method in which a terminal sends all possible transmission rates to an AP in advance in consideration of the application of a wireless LAN to home AV devices, and then performs stable transmission/reception upon setting the transmission rate to a relatively low rate if the channel capacity is large enough. In this method, when assigning a channel to a new terminal, the AP determines whether to accept a request from the new terminal, assuming that transmission/reception is performed at the maximum transmission rate applied by an existing terminal at the time of association. In this case, the maximum value of a usable transmission rate changes depending on a wireless channel state. However, there is no description about how to obtain the maximum value of a transmission rate in JP-A 2003-251791 (KOKAI).
Assume that an initial transmission rate is to be determined at the start of communication or the maximum transmission rate of each terminal in admission control is to be obtained. In this case, in consideration of the aggregation technique in IEEE802.11n which aggregates a plurality of frames and transmits the aggregated frame, depending on the number of frames to be aggregated, even if a transmission rate x is lower than a transmission rate y, the throughput at the transmission rate x may become higher than that at the transmission rate y. That is, in consideration of the aggregation technique, the assumption that the higher the transmission rate, the better does not always hold. It is therefore necessary to check the frame lengths (frame counts) to which frames can be aggregated at the respective transmission rates through all the transmission rates and compare them with each other.
As described above, according to IEEE802.11n, aggregation is performed to transmit an aggregated frame including a plurality of frames. In aggregation, a channel state is estimated from the preamble of the head of a frame, and an aggregated data frame is decoded following the preamble by using the estimated value. For this reason, a channel state corresponding to frames of the second half portion of the aggregated frame differs from the channel state at the time of estimation from the preamble, and hence errors generally tend to occur in the frames of the second half portion of the aggregated frame. The number of frames included in the aggregated frame depends on various factors, e.g., a transmission rate, a wireless channel state, and the type of decoding or channel tracking to be performed on the receiving side.
In general, the number of frames included in an aggregated frame at a high transmission rate, in particular, is smaller than that at a low transmission rate, it is not necessarily appropriate to assume that the higher the transmission rate, the higher the throughput. Under these circumstances, in consideration of both a transmission rate and the number of frames in an aggregated frame, it is necessary to select a combination of transmission rate and frame count which can obtain a high throughput. However, no studies have been made on a technique for selecting both a transmission rate and the number of frames in an aggregated frame from the viewpoint of throughput.